Method for controlling the carbon content in and/or the temperature of the steel

ABSTRACT

On the basis the experimental data, constants are predetermined to establish the decarburizing rate of a ferrous metal bath in a refining vessel in three successive oxygen blowing periods constituting the refining cycle. The model provided by the three equations determining the decarburizing rates is then used to control the refining time so as to obtain a steel of the desired carbon content from the charge of a given carbon content.

Inventors Takehiko Fujil;

Taijl Arakl; Katsukiyo Marukawa, all of Wakayama-shi, Japan Appl. No.843,884 Filed June 6, 1969 Patented Nov. 9, 1971 Assignee Sumitomo MetalIndustries, Ltd.

' Osaka, Japan Priority Nov. 27, 1965 Japan 40/73041 Continuation-impartof application Ser. No. 592,830, Nov. 8, 1966, now abandoned.

METHOD FOR CONTROLLING THE CARBON CONTENT lN AND/OR THE TEMPERATURE OFTHE STEEL 2 Claims, 3 Drawing Figs.

US. Cl Int. Cl Field of Search [56] References Cited UNITED STATESPATENTS 3,181,343 5/1965 Fillon 73/23 3,329,495 7/1967 Ohta et a1. 75/603,372,023 3/1968 Krainer et al 75/60 3.377.158 4/1968 Meyer etal 75/603,432,288 3/1969 Ardito et al... 75/60 3,455,164 7/1969 Boyle 75/60 UX3,463,631 8/1969 Vayssiere et al 75/60 Primary E.ranziner-L. DewayneRutledge Assislanl Examim'rG. K White Almrney-Kurt Kelman ABSTRACT: Onthe basis the experimental data, constants are predetermined toestablish the decarburizing rate ofa ferrous metal bath in a refiningvessel in three successive oxygen blowing periods constituting therefining cycle. The model provided by the three equations determiningthe decarburizing rates is then used to control the refining time so asto obtain a steel of the desired carbon content from the charge ofa75/60 C2lc5/32 75/60 given carbon content.

1 U I B 11 D C 15 U L 1 5 0'1: E w 8 5 J i 8 j 15 C E .9 3 o U E 8 q B C8 E E .L I P imestone f 5 a E I D .D E I! q? E T; 3 E E Time PATENTEUNUV9 Ian SHEET 2 0F 3 INVEN'IORS. IUJI l TAKEHIKO F TAIJI ARAK KATSUKIYOMARUKAWA AGENT PAIEN-TEUNDV 9 ml SHEET 3 [IF 3 FIG-3 DATA(|) 3 BEFOREINVENTOR8. TAKEHIKO FUJII TAIJI ARAKI KM K AGENT KATSUKIYO MARUKAWA BY mm l m w 0 a 29.551 wzirmm N E G VI X 0 F O T R .m S R L u F A METHOD FORCONTROLLING THE CARBON CONTENT IN AND/OR THE TEMPERATURE OF THE STEELREFERENCE TO RELATED APPLICATIONS The present application is acontinuation-in-part of our application Ser. No. 592,830, filed Nov. 8,1966, now abandoned.

This invention relates to a method for controlling the refining of steelin order to obtain molten steel having the desired carbon content,and/or to a method for controlling the temperature at the end point ofrefining steel.

The refining of steel comprises a decarburizing reaction in which thecarbon in the molten steel is separated in the form of CO or CO bycombining with oxygen in the atmosphere and simultaneously thetemperature of the molten steel is raised in order to remove anyimpurities. Therefore, it is possible to control the carbon content inthe molten steel and/or the temperature of the molten metal duringand/or at the end point of refining the steel by studying thedecarburizing reaction during the refining process and the mechanism ofthe refining process.

After having studied the decarburizing reaction, it has been found that,in the process of refining steel using an oxygen blown converter, therefining process may be divided into three blowing periods, the firstblowing period being the period in which the rate of the decarburizingreaction is mainly influenced by the temperature in the converter andthe silicon content of the material charged into the converter, thesecond blowing period being the period in which the rate of thedecarburizing reaction is mainly influenced by the quantity of oxygensupplied which is independent of the carbon content, and the thirdblowing period being the period in which the rate of the decarburizingreaction is mainly influenced by the carbon content.

It has further been found that the carbon content in the molten steeland/or the temperature of the molten steel during the refining processcan be calculated theoretically by setting up a model of thedecarburizing rate, as hereinafter defined, by making the temperatureincrease in the refining process correspond exactly to the decarburizingcondition and by making a model of the temperature increase of themolten steel, as hereinafter defined, corresponding to the model of thedecarburizing rate. The temperature model is of secondary importance anddepends on the model of the decarburizing rate.

Therefore, an object of this invention is to provide a method in whichthe refining time determines the desired value of the carbon contentduring the refining process and/or the carbon content at the end of therefining process, using predetermined formulas concerning thedecarburizing rate.

The predetermined model of the decarburizing rate and of the temperatureof the molten steel, or the refining conditions of the material to becharged into the converter is accordingly modified.

In the present invention, a model of the decarburizing rate is definedas a combination of variations of the decarburizing rates represented bythe formulas in the said three blowing periods and a model of thetemperature is defined as a combination of variations of the temperatureof the molten steel represented by the formulas on the basis of themodel of the decarburizing rate.

For a better understanding of the present invention and to show how thesame may be carried into effect, reference will now be made to theaccompanying drawing, in which FIG. 1 shows four relations between theblowing time and, respectively, the decarburizing rate, the amount ofreaction heat, the amount of solid material charged, and the temperatureof the molten metal;

FIG. 2 is a schematic showing of a steel-making plant incorporating acomputer control according to a preferred embodiment of the invention;and

FIG. 3 is a simplified flow chart depicting the method of thisinvention.

Referring now to the drawing and first to H0. 2, a ferrous metal chargeis placed into converter vessel 4 and the molten metal in the converteris converted to steel by blowing oxygen into the converter from thelance 3 which receives oxygen from supply line 1, the oxygen supplybeing controlled by valve 2 in the line. The exhaust gases from theconverter are removed through a hood placed over the mouth of theconverter and leading to an exhaust conduit in which the exhaust gasesare sprayed at 5 and then led through a Venturi tube 6, the exhaustgases being sucked through the exhaust pipe by blower 7 whose outputleads to stack 8. All of this structure and operation is substantiallyconventional.

While this is not essential, as will be explained hereinafter, thepresent method is practiced in the illustrated embodiment by means of acomputer 9 programmed according to the invention. As is shown in FIG. 2,the flow rate, pressure and temperature of the oxygen supplied to vessel4 is measured and the resultant signal is fed to the input of thecomputer through line A. The composition and temperature of the moltenbath in the vessel is sampled and the signal from this measurement isfed to the computer input through line B. The C0, C0 and 0 contents ofthe exhaust gas coming from the hood above the converter are sampled andmeasured, and the resultant signals are fed to the computer inputthrough line C. Finally, at Venturi 6, the flow rate, pressure,temperature and dew point of the exhaust gas are measured, and theresultant signals are fed to the computer input through line D.

The output of the computer, exemplified by lines x, y and z, controlsthe operation in dependence on the input signals. As a minimum, theoutput controls the oxygen supply and stops the same upon completion ofthe refining reaction. lt preferably also controls the oxygen flow rateby operating valve 2, the height of the lance, the addition of coolingand/or slag making agents to the molten bath, etc.

As used throughout this specification and the claims, the term model"refers to a set of equations mathematically expressing relevant refiningreaction conditions. Thus, a model of the decarburizing rate is a set ofequations representing the known relationships between the decarburizingrate and such factors as the refining time, the carbon and siliconcontent of the charge being refined, the amount of oxygen blown into thecharge, the refining temperature, the furnace configuration, and thelike.

Once, on the basis of experience, the coefficients sr constants of theequations of the model were set up for producing steel of a desiredcarbon content from a ferrous melt of given composition underpredetermined refining conditions, any refining reaction following thismodel and/or controlled to reflect the model will produce such steel.

In producing a model or prototype refining reaction, we have found thatthe reaction must be divided into three blowing periods.

FIG. 1 shows blowing periods I, ll and Ill corresponding to A B,BD and5B- In the first, blowing period I, the decarburizing rate (-dc/dr)varies with the refining time and depends on the silicon content and thetemperature of the molten material. ln other words,

where K, is a constant determined by the initial values of the siliconcontent and the temperature of the material in the converter, t is therefining time.

In the second blowing period II, the decarburizing rate is a function ofthe oxygen flow blown into the converter, or, if the oxygen flow is keptconstant during this period, is a fixed valve, i.e.

where K is a constant determined by the oxygen flow.

In the third blowing period 111. the decarburizing rate is a function ofthe carbon content or the velocity of the carbon arriving at thereaction surface, i.e. approximately ll/ a) where K,; is a constantdetermined by the oxygen flow, the lance height, etc. and

C is the carbon content of the molten steel bath.

The constants in the above equations, which form the proto type or modelof the decarburizing rate, are obtained experi mentally by running aseries of tests and averaging the results into a constant of desiredaccuracy. Obviously, this accuracy will increase with the number of testruns and the relevant data collected. The constants K K and K, are soselected as best to fit the refining vessel and the ferrous charge to berefined, the other parameters depending on the known refining conditionsof the reaction to be controlled by application of the model.

Apparatus and instruments for measuring and analyzing the oxygen flow,the amount and composition of the exhaust gas, the carbon content, thetemperature, the decarburizing rate and the like are well known, and anysuitable means of this type may be used in carrying out the method ofthe present invention. The total refining time and the duration of eachblowing period necessary to obtain a desired carbon content in the steelmay be calculated by suitable computers, also well known in the art. Anelectronic computer may be programmed with the models of thedecarburizing rate and the temperature, and the furnace operation isthen controlled in a known manner by the computer output to conform tothe models, as shown in FIG. 2.

FIG. 3 shows the sequential steps of the method. Computer 9 calculatesthe values of constants K,,', K and K,,, and the refining time, andafter the refining operation has started, it controls the oxygen blowingperiods. Before the refining operation starts, the computer effectuatesstages I and II.

The values of the constants are calculated on the basis of Data (1')which includes the predetermined oxygen flow rate, the silicon contentand the temperature of the ferrous metal to be refined, generalconverter factors, all based on the experimental data of past refiningtest runs.

In stage II, also before the refining begins, Data (ii) such as theoriginal carbon content of the ferrous metal charge, the desired carboncontent in the steel to be produced, and the values of the calculatedconstants, are substituted in the three equations setting up theconstants, to determine the total refining time needed to obtain thedesired carbon content in the steel. This is stored in the computer tooperate its control unit.

After stages I and II have been completed, the refining operation isstarted by blowing oxygen into the converter and the programmed computeris simultaneously set into operation to compare the stored model withthe actual operating conditions.

In its simplest form, the control unit of the computer simply produces astop signal H for halting the oxygen flow when the refining timecalculated in stage Ii has elapsed.

In the preferred procedure, sampling operations are performed at givenpoints throughout the refining reaction (S S ...S,,) to supply data tothe computer in stage Ill. The computer calculates the differencebetween the actual values sampled and the estimates derived from thestored model, and provides control signals as functions of thesedifferences to the control unit ofthe computer to provide controlcommands C C,,...C, to change the refining conditions accordingly, untilthe stop command H is produced.

In a simpler and less expensive way, the method of the present inventionmay also be carried out by (l) experimentally determining the threeconstants in the above equations covering each blowing period, (2)setting the desired carbon content of the steel to be obtained, (3)preparing a chart depicting the decarburizing rate model, as seen on topof FIG. 1,

(4) calculating the total refining time from the chart, (5) setting atimer to stop the refining reaction in the furnace at the calculatedtime, (6) start the refining operation, and (7) stop blowing oxygenunder control of the timer to terminate refining.

On the basis of experimental data, the three constants K K, and K aredetermined with respect to the refining vessel to be operated and theferrous melt to be refined in the following manner for any given point Pin the first blowing period I (see top of FIG. 1), successive points Rand Q in blowing period II, and any point S in blowing period III.

In the following equations, the decarburizing rate at a given point onthe curve on top of FIG. 1 is indicated as (dc/a'1)P, (dc/dr)B, etc. Therefining time counted from the start of oxygen blowing at each point ofrefining on this curve is indicated by l,,, 1,,, etc. The time elapsedbetween two successive refining points on the curve is indicated by 1 1etc. The carbon content of the melt at any refining point is indicatedby C C etc.

(A) PF? At the transition point B (a'c/dr),,=K the end of period Idc/dr,,=K the start of period II By equating the above two equations1,,=K /K, Therefore,

=K,-,/K r,. (B) From the equation 2 dc=K,-, dz; By integration Thedefinite integral from R to Q RQ 5( R u The definite integral from B toD nn a( n n) From the equation 3 dC/dFK,; C dc/C=K,, d!

By integration %=K fdt l/K,, log, =1 The definite integral from D to SME t ir s) (D) At the transition point D C=-K, 1 /2 The definiteintegral from P to B By considering the equation 4 n PB nn ns (G)Accordingly, if the values of I C, and C are measured by anyconventional means, and since the value C is a set value or a desiredcarbon value, the total refining time from point P to point B is easilycalculated from the equation 1 1.

However, if the values C and r,. are obtained, the value C is calculatedby the equation 10, and the value C is also obtained by the equation 9.

In summary, if the values C and 1,, are obtained, the total refiningtime t is easily calculated by the above equations.

Further, if the point P is shifted back to the point A, that is, thestarting point of the oxygen blowing, t,. becomes If or zero and C,.becomes C or the carbon value of the material charged which ismeasurable before the start of the refining operation.

Thus, equation above, becomes The above-detailed explanation ormathematical operation relates to the model of the decarburizing rate.However, since the model of the temperature is also represented by acombination of the three first order equations between the temperatureand the refining time for the respective blowing period, an analogousmathematical operation is applicable to the model of the temperature.

Generally speaking, the relationship between the temperature and therefining time or the carbon content in the molten steel is notcontrolling so that from the model of the temperature we could notobtain the total refining time or the carbon content in the moltensteel. The purpose of the temperature model is merely to assure whetherthe thermal reaction in the furnace is in the orderornot. H H V W Thereaction heat accompanying the decarburizing reac-' tion, and theoxidation of the silicon and the iron effect a temperature rise of themolten steel. However, as the reaction of the silicon and the iron inthe molten metal bath proceeds in quite intimate relation with thedecarburizing rate, it is necessary to monitor accurately thedecarburizing reaction mechanism in order to control the temperature ofthe molten. bath. r g H V 4 J In the first blowing period I, thedecarburizing rate does not" reach its highest level since thetemperature of the molten bath is still low and many impurities, such assilicon, manganese and phosphorus which are more easily oxidizable thanthe o xi ti ms asts l hsrsfme a m sta l hs. oxygen blown into the moltenmetal bath is used to oxidize the silicon, manganese and phosphorus,which oxidation generates a reaction heat. In the second blowing periodll, substantially all of the blown oxygen is used for decarburizing andthe temperature of the bath rises due to the decarburizing reactionheat. In the third blowing period ill, the carbon content decreased.Therefore, the blown oxygen exceeds the required amount due to thedecrease in the amount of the carbon arriving and diffusing at thereaction surface. Ac-, cordingly, blown oxygen begins to oxidize theiron and the amount of the decarburizing reaction heat decreases whilstthe amount of the iron oxidation heat increases.

As to the thermic reaction in the molten steel bath, if the exothermicreaction and the endothermic reaction correspond to the model of thedecarburizing rate, in the first blowing period I the pig iron, limestone and scale, etc. greater substantially with absorbing the heat offusion, and the greater part of the scrap melts till the end of thesecond blowing period ll. Some thermal dissipation through the brick ofthe furnace trunk and from the exhausted gas can be observed during therefining reaction. However, with the same furnace life, amount ofcharge, lance height and oxygen fiow, the thermal dissipation can betaken as substantially invariable. Therefore, if this thermaldissipation was previously measured by applying a standard charge intothe furnace and the result was substituted into the temperature model asa standard heat, the thermal dissipation can be neglected in consideringthe model of practical use.

As stated above, in the refining process of steel, the main factorsdominating the decarburizing rates are different from period to periodand the decarburizing rates naturally vary in accordance with theauxiliary factors, such as the oxygen flow blown in, the oxygen pressureor the reaction area, and the agitation effect of the molten steel dueto the lance position. Therefore, the decarburizing rate in each blowingperiod can be obtained more accurately by considering the auxiliaryfactors as well as the main factors. in table I the factors dominatin gthe decarburizing rates in the blowing periods are shown.

Generally speaking, the method of the present invention is carried outin the following sequence of steps:

1. the constants K K and K in the model or prototype of thedecarburizing rate are predetermined experimentally. 2. The desiredcarbon content of the steel at the end of the refining period isinserted in the equation 3. 3. Since the values or coefficients for thetotal refining time are thus determined, the values I or L arecalculated. 4. The refining operation is started. 5. The refiningoperation is continued for the duration I i.e. oxygen blowing is stoppedat the calculated time. in determining the constants K,, K and K theexperimental data concerning the main factors listed in table 1 areprimarily considered as mainly determinative of these constants.However, the above sequence of steps does not take into considerationthe auxiliary factors listed in table 1, such as the distance of thelance from the molten bath or the nozzle :configuration. if these arechanged during refining, the initially calculated time I becomesincorrect and must be adjusted. Therefore, greater accuracy will beachieved, if an additional step is inserted between steps (4) and (5),above to modify the constants in the light of changed refiningconditions and thus to change the total blowing time, or to modify therefining conditions in the light of a continuous comparison between therefining state at any given point calculated from the model of thedecarburizing rate and an analysis of the actuaLrefining state of thebath, i.e. its carbon content at such given point. The carbon content atany point may be readily measured by sampling the molten bath, and therefining times and/or decarburizing rate may also be readily measured atM 1299 1. is s Under many conditions, it will be necessary to take intoconsideration only the enumerated main factors since the auxiliaryfactors influence the refining reaction only minimally.

basis of a large number of experimental runs, the model will usuallyserve adequately without taking into account the auxiliary factors, andthe latter are not critical.

In the present invention, the model of the decarburizing rate in thethree blowing periods is prepared for the predetermined refiningconditions and the model of the variation of the temperature in themolten steel converter is prepared on the basis of the model of thedecarburizing rate. The model of the decarburizing rate is compared withthe actual decarburizing rate obtained continuously from the exhaust gasflow and the CO and CO contents in the exhaust gas or from the samplingof the carbon quantity of the steel converter during the refiningoperation till the end of the first or second blowing period or themodel of the decarburizing rate is compared with the decarburizing rateobtained during the second blowing period ll. The model of the variationof the temperature in the molten converter is compared with the actualtemperature in the molten converter at an arbitrary point during therefining operation by measuring the temperature several times. Anydifference between the model and the actual value is used to control thecarbon content in and/or the temperature of the steel converter duringthe refining of the charge or at the end of the refining, by modifyingthe model of the decarburizing rate of the charge, by modifying therefining condition so as to meet with the model of the temperaturevariation in the steel bath of the charge corresponding to modificationor coincidence of the model of the decarburizing rate, or by modifyingthe refining condition so as to meet the modification or coincidence ofthe model of the decarburizing rate.

Hereinafter, the method for modifying the refining condition in order tomodify the model of the decarburizing rate is explained precisely withreference to FIG. 1.

The model of the decarburizing rate is obtained on the basis of thesilicon quantity in the molten material charged into the converter andthe temperature of the material.

I. The time A B is read from a chart when the refining of thedecarburizing rate of the exhaust gas. The refining condition ismodified by the deviation between the actual time measured and the settime calculated, and the predetermined total time for refining ismodified.

2. The modification on the maximum decarburizing rate in the chartduring the second blowing period ii is effected by changing the refiningtime and the oxygen flow in order to obtain the desired carbon quantityin the molten steel to the effect that the proper refining condition isobtained after considering the propriety of the oxygen fiow and theoxygen content.

3. If the time PT) calculated from the model differs from the actualtime due to the inaccuracy in the carbon charge or the slopping under,the refining condition is modified by the dif-- ference so that thepredetermined refining time is modified or the carbon content at the endof refining can be assumed without actual analysis from the actualrefining time read afterthe blowing period III is over.

The refining conditions are so modified that the decarburizing rateactually calculated from the exhaust gas flow and the CO and CO quantityin the exhaust gas or the sampling of the carbon content in the moltensteelconvert er during the refining equals the set decarburizing rate inthe model, according to the following procedures:

1. if the actual increasing decarburizing rate in the blowing period Iof the charge increases over the set increasing decarburizing rate inthe model, the oxygen flow is decreased, and/or additional silicon orlimestone is charged as cooling agent. if the actual increasingdecarburizing rate decreases below the set increasing decarburizingrate, the oxygen flow is increased, and/or an oxygen source, such asiron ore, or an exothermic agent is charged into the converter.

2. If the actual decarburizing rate in the second blowing period ll ofthe charge increases over the set decarburizing rate in the model, theoxygen flow is decreased, the

oxygen lance height is increased, or the slag quantity is increased byadding a slag producing agent, such as limestone. If the actualdecarburizing rate decreases below the set decarburizing rate, theoxygen flow is increased, the oxygen lance height is decreased, or anoxygen source such as iron ore is charged.

3. [f the actual decreasing decarburizing rate in the third blowingperiod III of the charge decreases below the set decreasingdecarburizing rate in the model, the oxygen flow is increased, theoxygen lance height is decreased, or an oxygen source, such as iron ore,is charged. If the actual decreasing decarburizing rate increases overthe set decreasing decarburizing rate, the oxygen flow is decreased, theoxygen lance height is increased, or the slag quantity is increased byadding slag producing agents, such as limestone.

In steel refining plants, the steel is tapped from the furnace after thecarbon content and the temperature of the steel converter at the end ofrefining have reached the desired values. Therefore, the control of thecarbon content should be effected in addition-to the control of thetemperature in order to control the refining process.

In the present invention, the total refining time and the respectivetimes in the blowing periods, I, ll and Ill are calculated in accordancewith the system controlling the carbon content in the steel bath, thefactors of the control being the main material charged, the quantity ofthe material charged, the carbon content in the material charged, theoxygen flow and the lance height. Therefore, the model for the tempera-.ture of the steel bath in each blowing period can be calculated fromthe times referred to above and the process in which the temperature ofthe steel bath increases is made clear. In conclusion, the finishedcarbon content in and the temperature of the steel converter can becontrolled easily and accurately by measuring the temperature of thesteel converter during refining.

The following examples further illustrate the present invention withoutlimiting the same thereto:

EXAMPLE I tz a. Desired carbon content at the end of refining 0.07%Desired steel temperature at the end of refining 1,630 C. b. Measuredvalues:

Oxygen flow 23,000 Nm."/Hr. Main charge: Molten pig iron 128 tons Coldpig iron 7 tons Scrap 32 tons 167 tons Auxiliary charge: Limestone 7.0tons Scale 0.8 tons 7.8 tons Components of the molten pig iron:

C 4.45% Si 0.307: Mn 0.62% P 0.]941 Molten pig iron temperature at start1 212 C c. Time calculated on the basis of the model of thedecarburizing rate Refining time of blowing period I 7.00 min. Refiningtime of blowing period II 9.26 min. Refining time of blowing period Ill5.06 min.

Total refining time 2|.32 min. d. Temperature calculated from therefining time above Point B 1.252 C. Point D l,575C. Point E (desired)L630 C.

The above calculations were performed prior to the actual refiningoperation. e. The refining proceeded as follows:

1. After start, the recording of the decarburizing rate rate chart wasstarted. 2. Blowing period 1 coincided with the model so that noadjustment was needed.

EXAMPLE 2.

a. Desired carbon content at the end of refining (Llti'i percent Desiredsteel temperature at the end of refining 1.620 C.

b. Measured values:

Oxygen How ZLUUU NmifHr. Main charge: Molten pig iron I2l tons Cold pigiron 7 tons Scrap 4| tons I69 tons Auxiliary charge: Limestone 7.9 tonsScale |.3 tons 9.2 tons Components ofthe molten pig iron.

Si 0.47% Mn 0.69% P 0193" Molten pig iron temperature at start LJZ-tC.c. 'l'ime calculated on the basis of the model of the decarburiling rateRefining time of blotting period I 9.50 min. Refining time of blottingperiod II 7.40 min. Refining time ofblouing period lll 4.83 min.

Total refining time Zl.73 min. d. Temperature calculated from therefining time above Point 8 l.2hSC. Point D l.$6tt C. Point E (desired)lb L620 C. The ahoie calculations were performed prior to the actualrefining operation. e. The refining proceeded as follows:

I. Al'ter start. the recording of the decarburizing rate chart wasstarted.

2. Actual time to point D was 0.20 min. less than calculated. So thetotal refining time of the model was shortened by 0.20 min.

lTcmperature measured after refining of l7 min. 1.567 C. 4. Substitutedthe temperatures at 17 min. and at 4.53 min.

after the former measurement into the model 01 the temperature. 5.During 20 seconds after refining of l8.5 min. the refining as done withincreased lance height. 6. Actual refining time was extended by l0seconds from that of the model. 7. Analyzed carbon content of thefinished steel 0.17 ii. Temperature at the end of refining l.624 C.

known silicon content and a known carbon content C,,, b. blowing oxygeninto bath of the molten ferrous metal at a flow rate in successivefirst, second the third oxygen blowing periods;

LII

c. establishing said flow rate in the respective and successive blowingperiods by the following equations:

wherein K K;, and K are constants obtained on the basis of experimentaldata of past refining runs at said oxygen flow rate, the silicon contentand the temperature of the ferrous metal bath, and operating factors ofthe vessel, 1 is the refining time, C is the carbon content of themolten bath at any given point, dc/a'l being the decarburizing rate;

d. sampling the carbon content C in the first blowing period at a time1,. from the start ofthe oxygen blowing;

e. limiting the oxygen blowing time in the successive blowing periods torespective and successive times I,.,,. 1A,, and 1 totaling refining timeI from point P. these times being derived from the following equations,wherein the desired carbon content of the steel is C C being the carboncontent at a transition point B between the first and second blowingperiods. and C,, being the carbon content at a transition point Dbetween the second and third blowing periods:

wherein A 1 y 0B= (JP- mam) f-1P) and & C K6 and f. discontinuingrefining when the end of time I has been reached. 2. The method of claim1, wherein the time 1, of sampling the carbon content Cp is the start ofthe oxygen blowing whereby 1,. becomes zero in said equations.

2. The method of claim 1, wherein the time tP of sampling the carboncontent CP is the start of the oxygen blowing whereby tP becomes zero insaid equations.